I've been reading some interesting stuff related to D&D-type roleplaying games recently. One thing I came across is spell progression: How do you translate from the wizard or cleric level to the number of spells of each spell level the character can cast? Apparently different incarnations of D&D used somewhat different tables over the years.
All of this reminded me that I never liked the spell progressions in D&D-type games: I never "got" them in the sense that they always seemed way too random to me. What was the unifying mechanic behind all these numbers? Hard to figure out, go ahead, try.
I use the following simple progression: 1 2 2 3 3 3 4 4 4 4 5 5 5 5 5 and so on and so forth. Read: You start spell level x with 1 spell, after one more level you gain a second spell, after two more levels you gain a third, after three more you gain a fourth, etc. And I use this for each spell level. Both wizards and clerics gain a new spell level every 2 class levels, and done: One simple rule explains it all.
Yes, there is some “fluctuation” in terms of which character levels get the most new spell levels, but it’s really not too bad to say “look, level 18 is really powerful because that’s where you realize the most about how the multiverse really works". And yes, there are certain levels where you gain absolutely nothing, especially as a cleric, but those are really high and I wouldn’t normally play there anyway.